This invention relates generally to a control system for a water heater with an external heat source. More particularly, it is directed to a control system for a conventional domestic hot water heater used in association with an external heat source in the form of a heat pump or the like.
A conventional domestic hot water heater typically includes an internal heat source in the form of a tank heating unit having a heating element and a tank thermostat. When the thermostat indicates a demand for hot water, the heating element is turned on. When the thermostat indicates that the demand for hot water has been satisfied, the heating element is turned off.
The heat pump has been known for many years. Although it is more economical to operate than a conventional tank heating unit, it has not been used extensively heretofore because power, particularly electric power, has been inexpensive. As the cost of power increases, there is renewed interest in the use of a heat pump in association with a water heater, particularly for providing domestic hot water.
When a heat pump is provided for use with a hot water heater, it would be advantageous if its control system were responsive to the tank thermostat as an indicator of the demand for hot water. Such an arrangement is disclosed in copending U.S. application Ser. No. 640,891 filed Aug. 14, 1984, which is a continuation of U.S. application Ser. No. 416,435 filed Sept. 10, 1982 now abandoned. This application is of common assignee herewith, and is incorporated herein by reference. A problem with this arrangement is that it requires that the tank heating unit be rewired. This would be acceptable in an equipment package for the new construction market. However, it would not be acceptable in an aftermarket package for on-site installation in association with a conventional domestic hot water heater. The problem is that rewiring the tank heating unit in the field would negate the Underwriters Laboratories approval.
Without modification of the tank heating unit, the water pump within the heat pump could be cycled on periodically, and a measurement could be taken of the temperature in a line communicating the hot water tank with the heat pump. The problem with this, however, is that it would shorten the working life of the water pump.
A different problem may occur when a heat pump is provided for use with a hot water heater. There may be occasions when an external condition would render the heat pump ineffective and/or inefficient. Such an external condition might be, for example, the formation of frost on the evaporator of the heat pump, in which case its heating capacity would not remain adequate to meet the hot water requirements.
One solution would be to provide the heat pump with a defrosting capability. However, the problem here is that this would add significantly to the cost.
Another solution is disclosed in the aforementioned U.S. application Ser. No. 640,891 wherein an evaporator thermostat is provided, and wherein the heat pump is turned off and the tank heating unit is turned on when a frost condition at the evaporator is indicated. Again, however, this arrangement requires that the tank heating unit be rewired, and thus would negate the Underwriters Laboratories approval.
There remains a need in the art for a control system for a hot water heater with an external heat source, which system is conducive to installation on-site without modification of the existing hot water heater, and which will solve the problems noted above.